It confirms that Altair, one of the brightest stars in the night sky, is a rapidly spinning, non-spherical body.

Until now, telescopes have only been powerful enough to zoom in on the Sun or on rare giant stars outside of the Solar System.

But researchers, writing in the journal Science, say they got around this problem by combining the light from four separate telescopes.

We're testing the theories of how stars work in much more detail than ever before

John Monnier

Lead researcher John Monnier, an astronomer at the University of Michigan, US, said: "What may not be obvious to most people is that until now we have not really had the zoom-up capability with our telescopes to make images of stars that are like the Sun."

Compared with the Sun, which can be imaged in spectacular detail, and some rare gas giants, stars outside of the Solar System simply appear as specks of light through even the most powerful telescopes.

Professor Monnier said: "Even Altair, which is pretty close at 15 light-years away and very bright, would be very challenging to zoom-up on. In order to do this, you would need a telescope that is about 300m (1,000ft) across - and that is a long way beyond our engineering capabilities."

Elongated shape

To image Altair, which is the brightest star in the Aquila constellation (The Eagle) and clearly visible to the naked eye, the researchers harnessed the power of four telescopes at Georgia State University's Center for High Resolution Angular Astronomy (Chara).

Altair appears as a speck from other telescopes

Professor Monnier told the BBC News website: "By combining and processing the light from the four smaller telescopes, using a new instrument called the Michigan Infrared Combiner, we are creating an image as if it is coming from a much bigger telescope."

The result was a detailed surface image of Altair.

Previous research had suggested that this bright star was spinning very rapidly - about 60 times faster than our home star. But until now, the effect of the rapid rotations on its shape was not clear.

Professor Monnier said: "Computer models had given very basic predictions of what happens if you have a star that is spinning very fast, but our image definitely confirmed that this star was elongated in shape."

The Sun has been imaged in spectacular detail

The centrifugal forces created as the star was spinning were flattening it into an oval shape, he said.

However, the image also revealed that the amount of distortion and changes in surface temperature at the equator differed from current models.

Professor Monnier said: "Since we have a lot more data and the images are very powerful, we can do the modelling again to give a new level of detail.

"We're testing the theories of how stars work in much more detail than ever before."